QAM mode transmitters generate a radio frequency (RF) signal that is formed by adding modulated sinusoid and cosine carriers, which are also referred to as in-phase (I) and quadrature (Q) signals, respectively. The I and Q signals can be digitally generated at a lower frequency and then upconverted to a carrier frequency. The I and Q signals can be upconverted with a pair of analog mixers—one mixer for the I signal and the other mixer for the Q signal. The analog mixers are generally less expensive than their digital counterparts and therefore more popular.
In an ideal case, the amplitudes of the unmodulated I and Q signals are equal and the I and Q signals are exactly 90° out of phase with each other. However, variances in the analog mixer pairs distort or mismatch these relationships. The mismatch is referred to as I/Q mismatch. Since a receiver may incorrectly interpret the I/Q mismatch as an information signal, it is important for the transmitter to minimize the I/Q mismatch.
Referring now to FIG. 1, a functional block diagram is shown of an I/Q-mismatch compensated transceiver 10 according to the prior art. Transceiver 10 includes a transmitter section 12, a receiver section 14, and an I/Q mismatch calibration section 16.
Transmitter section 12 includes an I/Q predistortion module 24 that compensates the magnitude and/or phase relationship of I and Q signals. The compensation is based on an amplitude correction signal αest and a phase correction signal βest, referred to collectively as correction signals, that are generated by calibration section 16. I/Q predistortion module 24 compensates for the I/Q mismatch that may be introduced by a pair of analog mixers included in an analog transmitter 30.
During a calibration sequence, a loopback switch 44 is closed and couples the output of transmitter section 12 to an input of receiver section 14. Calibration section 16 then measures the I/Q mismatch introduced by the mixers in analog transmitter 30. Calibration section 16 generates the correction signals based on the measurement. I/Q predistortion module 24 then compensates the magnitude and/or phase relationship of the I and Q signals to eliminate the I/Q mismatch at the outputs of analog transmitter 30.
A transmit filter module 26 filters harmonics from the I and Q signals. Outputs of transmit filter module 26 communicate the I and Q signals to respective inputs of a digital-to-analog converter (DAC) 28. DAC 28 converts the digital I and Q signals to corresponding analog signals. The analog I and Q signals communicate with respective inputs of analog transmitter 30.
Receiver section 14 includes an analog receiver 32. Analog receiver 32 includes a second pair of analog mixers that regenerate the I and Q signals from the RF carrier. The second pair of analog mixers introduces additional I/Q mismatch into the received I and Q signals. An analog-to-digital converter (ADC) 36 converts the analog I and Q signals into digital I and Q signals. A receive low-pass filter (LPF) 36 filters harmonic frequencies and communicates the filtered I and Q signals to a receive I/Q compensation module 38. I/Q compensation module 38 compensates the digital I and Q signals based on the correction signals from calibration section 16 and compensates for the I/Q mismatch that was introduced by analog receiver 32.
Based on a DO_CALIB signal, a demultiplexer 40 routes the compensated I and Q signals to calibration section 16 or a carrier recovery module 42. The DO_CALIB signal, and a CALIB_MODE signal that is used by a demultiplexer 50, are asserted while transceiver 10 is being calibrated for I/Q mismatch.
Calibration section 16 includes an I/Q calibrator module 48 that measures the I/Q mismatch between I and Q signals that enter calibration section 16. I/Q calibrator module 48 then generates the correction signals based on the I/Q mismatch. Based on the CALIB_MODE signal, a demultiplexer 50 then routes the correction signals to transmit I/Q predistortion module 24 or receive I/Q compensation module 38.
Operation of transceiver 10 will now be described. Transceiver 10 supports three operating modes—a receiver calibration mode, a transmitter calibration mode, and a normal operating mode. The I/Q mismatch calibration process begins in the receiver calibration mode.
In the receiver calibration mode loopback switch 44 is opened, a switch 46 is closed, and the DO_CALIB and CALIB_MODE signals are set equal to “1”. Switch 46 connects the input of analog receiver 32 to a source 52. Source 52 generates a reference RF carrier that includes ideal I and Q signals. The analog mixer in analog receiver 32 introduce receiver I/Q mismatch to the ideal I and Q signals. I/Q calibrator module 48 measures the receiver I/Q mismatch and based thereon generates the correction signals. Demultiplexer 50 routes the correction signals to receive I/Q compensation block 38. I/Q compensation block 38 stores the correction signal values and thereafter compensates the received I and Q signals to eliminate the receiver I/Q mismatch.
Transceiver 10 then enters the transmitter calibration mode. In the transmitter calibration mode switch 46 is opened, loopback switch 44 is closed, the DO_CALIB signal is set equal to “1”, and the CALIB_MODE signal is set equal to “0”. Since receiver section 14 has already been compensated, I/Q calibrator module 48 can measure the transmitter I/Q mismatch and generate the correction signals for transmitter section 12. Multiplexer 50 routes the correction signals to I/Q predistortion module 24. I/Q predistortion module 24 stores the correction signal values and thereafter compensates the I and Q signals to eliminate the transmitter I/Q mismatch. The normal operating mode can then be entered by opening loopback switch 44 and switch 46, and setting the DO_CALIB and CALIB_MODE signals equal to “0”.
Referring now to FIG. 2, a transmitter 60 is shown that employs an alternate method of generating the correction signals for I/Q predistortion module 24. A spectrum analyzer 62 monitors the RF carrier signal while known data signals are communicated into IFFT module 18. Spectrum analyzer 62 then measures the I/Q mismatch introduced by analog transmitter 30 —and generates the correction signals accordingly. I/Q predistortion module 24 stores the correction signals and compensates the I and Q signals accordingly.
The above methods calibrate transmitters 12 and 60 for I/Q mismatch at a single frequency. The methods can be repeated to calibrate for I/Q mismatch at a number of frequencies. For example, unique correction signals can be generated for corresponding RF bands and/or frequencies used by the Institute of Electrical and Electronics Engineers (IEEE) standards 802.11, 802.11a, 802.11b, 802.11g, 802.11h, 802.11n, 802.16, and 802.20, and/or used by the Bluetooth Special Interest Group (SIG) Bluetooth standard. The aforementioned standards are hereby incorporated by reference in their entirety. I/Q predistortion module 24 can then store the plurality of correction values and use the correction values associated with the carrier frequency being used.
While the above circuits and methods address the issue of I/Q mismatch, they include some undesirable aspects. For example, the method used with transmitter section 12 is dependent on calibrating receiver section 14. If receiver section 14 is improperly calibrated then the error will adversely affect the correction signals that are generated for transmitter section 12. Transmitter 60 of FIG. 2 overcomes the issue by calibrating independently of a receiver section; however it can take an unacceptable amount of time to connect the spectrum analyzer and generate the known data signals.